DE212012000091U1 - pump rotor - Google Patents

Abstract

A vacuum pump rotor for use in a vacuum pump with a Roots pump mechanism, the rotor having at least two hollow cams, each cam having an outer wall that defines a cam profile, a cavity generally within the outer wall, and at least one reinforcing rib that is in the cavity is arranged to withstand stress on the cams generated during rotation.

Description

The invention relates to a rotary positive displacement pump and a rotor of such a pump. In particular, the invention relates to Roots pumps (also known as Roots blowers).

Roots pumps typically include a pair of intermeshed cammed rotors that revolve in a housing and cause fluid to be trapped in pockets surrounding the cams and conveyed from the pump inlet to the pump outlet. The rotors do not really touch each other, so no lubricant is required. This makes Roots pumps desirable in applications where contamination of the fluid poses a problem, for example in semiconductor processing.

A simplified schematic of a typical roots pump 100 is in 1 shown. A pump chamber 101 is by a plurality of stator components including a stator housing 102 and two transverse end walls 104 educated. The end walls 104 have openings 106 through which two rotor shafts 108 . 110 run. The waves are at each end by bearings 112 supported. An engine 114 drives the rotation of a shaft 108 on, and a transmission mechanism 116 transfers the torque to the other shaft 110 , The gear mechanism causes the shafts to rotate in opposite directions in synchronization with each other.

On the waves are pairs of rotor-cams 118 . 120 and 122 . 124 assembled. The radial tip of the cam 122 is through the cam 120 hidden and therefore marked with dashed lines. 2 shows a section through a pump along the line II-III in which the rotor socks are clearly visible. As the rotors rotate, the cams stroke the inner surface 122 the pump chamber 101 passing and thereby pumping fluid from the chamber inlet 128 to a chamber outlet 130 , The tolerances between the rotor cams and the painted surface 126 must be tightly controlled, as well as the tolerances between the rotors, otherwise gaps are created through which fluid can pass, reducing the efficiency of the pump. Typical tolerances are in the range of 0.1 millimeters.

Typical Roots pumps have an acceptably high pump capacity, but for some applications it is desirable to further increase the capacity of the pump. This can be achieved while maintaining the cam tip speed by providing a larger pump with larger cams. However, this is disadvantageous in that the pumps become more expensive, and when an accident occurs, for example, when the rotors collide, the increased energy of the cams may be sufficient for the cams to break through the pump housing and cause damage or injury.

Alternatively, the capacity of the pump can be increased by allowing the rotors to spin faster. A typical cam tip speed during orbit is less than 100 m / s, and often less than 80 m / s. Significantly increasing the speed at the tip of the cams to, for example, 130 m / s would make it possible to make the cams smaller and reduce the cost of the pump. However, even if the cams are less massive, the increased speed causes an increase in cam energy, and damage or injury may also be caused in the event of an accident. It should also be noted that increasing the speed causes a greater increase in kinetic energy than an increase in mass because the energy is proportional to mass but proportional to the square of the speed.

Conventional rotors are usually made of a solid block of material, typically cast iron. Such rotors can be made in a variety of ways, including casting solid cams and shaft integrally with one another, or casting solid cams and securing the cams to a shaft to form the rotor.

Known cams can be made by casting a solid cam and then drilling a hole therein to reduce its weight.

The present invention intends to increase the pump capacity of such rotary positive displacement pumps by further reducing the weight of the rotors for a given pump size. The present invention also aims to solve known problems of using hollow cams, in particular the problems of ensuring that the camming walls remain strong enough to withstand operational stresses and not to deform out of tolerance.

According to the present invention, there is provided a vacuum pump rotor for use in a vacuum pump having a roots pump mechanism, the rotor having at least two hollow cams, each cam having an outer wall defining a cam profile, generally within the outer wall hollow space, and having at least one reinforcing rib within the cavity to withstand stresses generated during rotation on the cams.

The or each reinforcing rib may extend along an inner wall of the cams. The or each reinforcing rib may have a variable extent and may be distributed within the cavity depending on the varying stresses applied to the lobes during operation.

The outer wall may have a variable thickness and is thicker at a radially inner portion than at the cam tip.

The outer wall of the cams may have a thickness such that the cams deform under centrifugal load when the rotor is rotated during operation, and the deformation is greater than manufacturing tolerances. The cam profiles may have an optimal configuration in a first state in which the rotor is rotated during operation, and in a second state when the rotor is not rotated and the cam profile is not in an optimal configuration, and wherein the cam is out deformed to the second state in the first state, when the peak speed of the cam is greater than 100 m / s.

Preferably, the ratio of the thickness of the wall to a radius at the cam tip is less than 1:20. The wall thickness can be less than 5 millimeters if the radius of the cam tip is at least 100 millimeters.

Each hollow cam may have a plurality of hollow cam portions which are connected in axial sequence along the rotor and together form the cam.

Each of the hollow cam portions may have a flange extending circumferentially and radially inwardly around at least one axial end of the portion to interconnect adjacent portions.

One or more holes may be provided in the flanges to fasten the hollow cam portions together by fasteners.

Each cam may further include two end surfaces for closing the cavity at each axial end of the cam.

The rotor may have a shaft, and the cams may include means by which the cam may be secured to the shaft, the cams and the shaft being shaped to form an approximately continuous profile of the at least two cams and the shaft.

The hollow cam and the shaft of the rotor may be adapted to be assembled by means of a dovetail or similar connection so as to minimize the radial movement of the hollow cam portion with respect to the shaft of the rotor.

The rotor may include venting means for allowing the pressure within the cavity to substantially equalize with the pressure outside the hollow cams. The venting means may include a filter for filtering out deposits of gas carried by the venting means into the cavity.

The invention also includes a vacuum pump with a rotor as described above.

The pump may include a plurality of pump stages each having a pumping chamber and at least two cams.

At least one of the pump stages may include a cam having a plurality of cam portions connected in axial sequence.

The reinforcing ribs in the cam cavities may extend in respective radial planes relative to the axes of the rotor shafts, and the radial planes of the lobes of one rotor are out of alignment with the radial planes of the other rotor.

The portions of the cams between radial planes may be arranged to deform when the radial plane aligned portions of the cams strike to absorb energy of the rotors in the event of a random rotor impact.

The present invention also provides a method of manufacturing a rotor for a vacuum pump, the method comprising providing the rotor with at least two hollow cams, each cam having an outer wall defining a cam profile and a cavity substantially within the outer wall and wherein within the cavity at least one reinforcing rib is arranged to withstand the stress generated on the cams during rotation.

The present invention will now be described with reference to the accompanying drawings, in which:

1 a schematic representation of a known Roots pump shows;

2 a cross section through the known Roots pump after 1 shows;

3 a schematic representation of a Roots pump according to the present invention shows;

4 a cross section through the Roots pump after 3 shows;

5 a cut out view of a hollow cam shows the part of the in the 3 and 4 represented Roots pump forms;

6 an isometric view of a hollow cam portion according to the present invention and an end plate shows;

7 a cross section through a rotor with hollow cam sections, as in 6 shown, shows;

8th showing deformed and undeformed states of a red-tailed skirt;

9 a schematic representation of a multi-stage pump according to the present invention shows;

10 shows a modified arrangement of the reinforcing ribs in the cam cavities of the rotors.

The 1 and 2 show a typical roots pump. These figures are described above in the introductory part of this document as they form part of the prior art.

3 shows a pump according to the present invention. Some features are common to both the invention and the known pump, and these features are designated by common reference numerals. A pump 150 has a pump chamber 151 due to a plurality of stator components including a stator housing 102 and two transverse end walls 104 is formed. The end walls 104 have openings 106 through which two rotor shafts 108 . 110 run. The waves are at each end by bearings 112 supported. An engine 114 drives the rotation of a shaft 108 on, and a transmission mechanism 116 transfers the torque to the other shaft 110 , The gear mechanism causes the shafts to rotate synchronously in opposite directions.

On the waves are pairs of rotor cams 160 . 162 and 164 / 166 assembled. In this schematic illustration, the rotors are shown in a configuration to assist in describing the embodiment of the invention to form thin walls 208 and cavities 210 to show. All rotor cams are hollow, each cam has a thin curved outer wall 208 that have a cavity 210 surrounds. Furthermore, all rotor cams are of axially modular construction. The thin wall 208 has a thickness in a ratio of less than 1:20 to the tip radius of the cam. Preferably, the ratio is less than 1:40, and more preferably about 1: 100. For a pump with a cam tip radius of 200 millimeters, the thickness is preferably less than 10 millimeters, more preferably less than 5 millimeters, and ideally about 2 millimeters to 4 millimeters thick. In this example, each cam is formed of three hollow cam sections, although 2, 4 or more hollow cam sections may be used instead, depending on the desired axial length of the rotor. The cam 166 is made of hollow cam sections 202 . 204 and 206 and two end plates 212 formed with an end plate disposed at each axial end of the cam. The hollow cam portions may be of equal axial length or may be of different axial lengths. For ease of manufacture, it is usually desirable to use hollow cam sections of equal axial length. In this example, the hollow cams are made of alloy steel for high strength and good temperature resistance. Other materials such as aluminum could be used instead. In addition, the hollow cam portions can be made by other known manufacturing methods. The hollow cam sections have a flange 214 . 216 at each axial end to allow the coupling of the hollow cam portions together. This will be more specific with respect to 5 described. Alternatively, the flange 214 . 216 on an end plate 212 be fixed when the hollow cam portion to be arranged at an axial end of the rotor.

4 shows a section through the pump 3 along the line AA, where the hollow rotor socks are more visible. In the 4 shown rotors are not in the same configuration as in 3 as shown by knowledge of Roots pumps. As the rotors rotate, the hollow cams sweep 160 . 162 . 164 . 166 on the inner surface 126 the pump chamber 151 passing and thereby pumping the fluid from a chamber inlet 128 to a chamber outlet 130 ,

5 shows the connection between the hollow cam sections 202 and 204 more in the individual. The hollow cam section 202 has a flange circumferentially and radially inwardly around the axial end of the hollow cam portion 202 runs. Similarly, the hollow cam portion 204 a flange 216 which extends circumferentially and radially inwardly around the axial end of the hollow cam portion 204 runs. The flange 216 has a lip 215 that allows the flange 216 the flange 214 overlaps. A hole (in 6 shown) is provided in each of the flanges to attach the hollow cam portions 202 . 204 using a screw 220 to enable. It is important that these holes be properly aligned so that the outer walls of the hollow cam sections remain flush with each other when they are bolted, and so that the connection as far as possible the inner cavity 10 against the pump chamber 151 seals. In order to ensure that a fluid-tight seal is achieved, a sealant may additionally be provided at the connection. The lip 215 is optional, but it helps to achieve a well-sealed connection. If there are manufacturing or other restrictions, the lip can be omitted. In this case, the flange has 216 the same shape as the flange 214 , and the flanges can be bolted together as described above.

6 shows a hollow cam portion 204 more in detail. A thin outer wall 208 forms a cavity 210 which is open at both axial ends. reinforcing ribs 226 are provided to provide increased strength to the thin outer wall to withstand stresses when the pump is in operation and to maintain the desired profile of the cams in operation. The ribs extend around the inner peripheral surface of the curved wall 208 and are distributed according to the expected loads during the circulation. In this regard, it can be seen that the amount by which the ribs project from the inner wall of the cam varies over the peripheral nature of the cam. The radially inner part of the ribs projects most where the working stresses are highest, and as the stresses decrease, the ribs spring to a lesser extent. The ribs are with holes 224 provided to balance the rotor, for example by adding screws and nut. Alternatively, the holes may be drilled at appropriate locations on the ribs to remove mass and thereby balance the cams. At each axial end of the hollow cam portion extends a flange 214 circumferentially around the inner surface of the curved wall. The flange 214 can be identical to the reinforcing ribs for ease of manufacture 226 be. holes 222 are provided to screw the flange 214 to allow with the flange of an adjacent hollow cam portion, as in 5 is shown. Alternatively, the flange 214 on an end plate 212 bolted when the hollow cam portion is disposed at an axial end of the rotor. An end plate 212 is in 6 shown and shaped so that it fits into the recess, through a wall 213 in flange 214 is formed. The end plate 212 has a through hole 227 so that the cavity of the cam can be vented and the pressure in the cavity can equalize with the pressure in the pump chamber. If the gas pressure in the cavity is greater than in the pump chamber due to insufficient sealing, the gas leaks out of the cavity and reduces pump efficiency. A filter medium 225 such as a glass fiber mat prevents solid deposits generated from a pumped process gas from entering and accumulating in the cavity. Accumulated deposits would increase the cam mass and lead to an unbalance of the cam.

High strength screws 230 and corresponding holes (not shown) are provided to allow the hollow cam portion to be bolted to the rotor shaft. A swallowtail 228 is also provided which fits into a complementarily shaped groove in the rotor shaft to form a dovetail joint. The dovetail connection is useful because it aids alignment of the hollow cam portions during assembly of the cam. Further, it also provides a safety system in that, upon breakage of the screw fixing the hollow cam portion to the rotor shaft (eg, shearing due to fatigue or a rotor impact), the dovetail joint prevents the cams from disengaging from the rotor shaft and can cause serious damage.

7 shows a pump rotor with two hollow cams 164 and 166 and a rotor shaft 110 , The hollow cams are hollow cam sections 204a . 204b formed and each have thin curved walls 208a . 208b that have a cavity 210a . 210b enclose. A flange 214a . 214b is provided to secure the hollow cam portion to either another hollow cam portion or to an end plate as needed. holes 222 are in the flanges 214a . 214b provided to facilitate the attachment. High strength screws 230 are used to make the hollow cam sections with the rotor shaft 110 to screw. The hollow cam sections each have a dovetail 228a . 228b which fits into a complementarily shaped groove in the rotor shaft to form a dovetail joint.

The configuration of the cams, which have a thin wall and a cavity, reduces the mass of the cams while maintaining the outer cam profile. As the mass is reduced, the rotors can revolve faster without increasing the amount of energy stored in the rotating cams. For example, the rotors may rotate at a cam tip speed of greater than 100 m / s, and preferably about 130 m / s. In known designs, rotating the rotors at such speeds would increase the stored energy in the rotors above acceptable limits, with the risk of damage or injury in the event of an accident. It should also be noted that rotating a thin-walled hollow cam at speeds of about 130 m / s requires the use of the reinforcing ribs discussed above, which are necessary to absorb the increased stresses on the cams. Even with the reinforcing ribs, the cams deform at high rotational speeds due to centrifugal loading. The deformation caused thereby is greater than manufacturing tolerances. In this regard, the deformation at the cam tip can be 0.5 to 1 millimeter, while manufacturing tolerances can be 0.1 to 0.2 millimeter. Therefore, embodiments of the present invention are designed so that the cams assume an optimal pumping condition when rotated at high speeds. That is, the cams deform under centrifugal load at high speeds to achieve optimum configuration. Known pumps deform under load but less than manufacturing tolerances of, for example, 0.1 to 0.2 millimeters.

It necessarily follows that at low speeds, the hollow cams are not in an optimal pumping state and therefore there are gaps between the cam profiles and between the cam profiles and the swept surface of the pumping chamber. These gaps cause leakage and reduce pump efficiency, but the reduced efficiency at low speeds is an acceptable disadvantage for increased pumping at high speeds.

8th Figure 3 shows in solid lines a non-deformed state of a hollow cam when the pump is stationary and in dashed lines the outer profile of the cam when it is in a deformed state and the pump rotates at high speeds. In the 8th shown deformation is greatly exaggerated for the purpose of explanation.

More in detail, the cam deforms radially outward at the cam tip 264 when the cam is stretched under centrifugal force. The cam pages 265 deform inwards towards the center of the cam. The wall thickness of the cam varies and is thicker on the sides than on the cam tip, which helps to avoid the greater stresses on the cam towards the center of rotation which decrease radially outward. Likewise, the reinforcing ribs project to a greater extent into the cavity at the cam base and to the side than at the cam tip.

This cam configuration allows much thinner cam walls (and therefore lighter weight cams) to be used than if a non-deforming design were used. Furthermore, the rotor shaft 110 designed to complete the outer profile of the hollow cam portions when the pump is in operation to produce an optimum profile for the rotor as in 7 is shown.

9 shows a multi-stage pump 300 with four pump chambers 308 . 306 . 304 and 302 , The first pump chamber 308 has three hollow cam portions connected to each other to form each cam, the second pump chamber 306 has two hollow cam portions joined together to form each cam, and the third and fourth pump chambers 304 . 302 have only one hollow cam section per respective cam. Within each of the pump chambers, each of the cams has an end plate 212 at each axial end, leaving the cavity 210 within each cam is completely completed. Each of the pump chambers is defined by a plurality of stator components including a stator shell 102 and two transversal end walls 104 educated. The end walls 104 have openings 106 through which two rotor shafts 108 . 110 run. The waves are at each end by bearings 112 supported. An engine 114 drives the rotation of a shaft 108 on, and a transmission mechanism 116 transfers the rotational power to the other shaft 110 , The gear mechanism causes the shafts to rotate synchronously in opposite directions.

The pump chamber 308 is similar to the pump chamber 151 , in the 3 and is shown schematically to describe the invention. Inside the pump chamber 308 are rotor shafts 108 . 110 with pairs of rotorcocks mounted on it 160 . 162 . 164 . 166 , All these socks are hollow, each sock has a thin, curved outer wall 208 that have a cavity 210 surrounds. Furthermore, all rotor cams are of axially modular construction. The thin wall 208 is about 2 millimeters to 4 millimeters thick. In this example, each cam is formed of three hollow cam sections, although 2, 4 or more hollow cam sections may instead be used, depending on the desired axial length of the camshaft Rotor. The hollow cam sections have flanges 214 . 216 at each axial end to allow the mounting of the hollow cam sections together. This will be more specific with regard to 5 described. Alternatively, the flange 214 . 216 on an end plate 212 fixed when the hollow cam portion is disposed at an axial end of the rotor.

The pump chamber 306 is similar in construction to the pump chamber 308 , except that the axial length of the chamber 306 shorter and therefore only two hollow cam sections are required to form each cam. Similarly, the pump chambers 304 . 302 similar in construction to the pump chambers 308 . 306 with the exception that their axial lengths are shorter and therefore only one hollow cam section with two end plates 212 is required to form each cam.

The end walls 104 , which are arranged between the pump chambers, the pump chambers separate from each other and are adapted to flow fluid from the outlet of an upstream pump chamber into the inlet of the adjacent downstream pump chamber. The end walls 104 , which are located at each axial end of the pump stack, separate the pump stack from two other components of the pump, such as the transmission and motor, and are adapted to introduce fluid into the inlet of the first (the most upstream) pump chamber 308 and from the outlet of the last (most downstream) pump chamber 302 to let escape.

In operation, each of the pump chambers acts to pump fluid from its inlet to its outlet. The outlet of a pump chamber is above an end wall 104 in fluid communication with the inlet of the adjacent downstream pumping chamber so that the compression achieved by the pump is cumulative.

Four pump chambers are in 9 but more or fewer pump chambers can be used as needed. The pump chambers, which are in 9 have the same diameter, but if desired, the pump chambers may have different diameters from each other. Furthermore, each pump chamber itself need not have a constant diameter, but may be conical.

All the above examples show the end surfaces 212 separated from and connected to the hollow cam portions to form the closed hollow cam. Alternatively, one of the end faces 212 be formed integrally with the hollow cam portions. Ideally, the axial length of the hollow cam portions should be chosen to optimize the manufacturing process so that the hollow cam portions, including their flanges and ribs, can be easily machined and assembled. Furthermore, the axial length of the hollow cam portions is ideally not too long, otherwise access to the screws connecting the hollow cam portions to the rotor shaft may be limited.

10 shows a modified arrangement of reinforcing ribs in respective rotors 402 . 404 for a single-stage pump. The arrangement is also applicable to multi-stage pumps. For purposes of this discussion, the two rotors are shown separated from one another, whereas in practice the cams would overlap, as more fully described above.

The rotor 402 has cams 403 . 405 on, and the rotor 404 has cams 407 . 409 on. The reinforcing ribs 406 . 408 of the rotor 402 are each in cam cavities 410 . 412 are located and extend in radial planes R1, R3, R5, R7, R9, R11 and R13 relative to the axis A1. The reinforcing ribs 414 . 416 of the rotor 404 are in respective cam cavities 418 . 420 arranged and extending in radial planes R2, R4, R6, R8, R10 and R12 relative to the axis A2. The radial planes R1, R3, R5, R7, R9, R11 and R13 of the motor 402 do not align with the radial planes R2, R4, R6, R8, R10, and R12 of the rotor 404 , It is understood that the parts of the cams which are in alignment with their supporting reinforcing ribs are stronger than the parts of the cams which lie between the reinforcing ribs in the axial direction. For example, with reference to the drawing, a part 422 of the cam 405 , which is approximately aligned with the radial plane R3, is stronger than a part 424 which lies between radial planes R1 and R3. In the same way is a part 428 of the cam 407 , which is approximately aligned with the radial plane R2, stronger than a part 430 which lies between radial planes R2 and R4. The stronger part 422 of the cam 405 Aligns with the deformable part 430 of the cam 407 , and the stronger part 428 of the cam 407 Aligns with the deformable part 428 of the cam 405 , Accordingly, in the case of a high-speed collision between rotors, the deformable parts of a cam are deformed by the strong parts of the other cam, thereby absorbing the high stored energy of the rotors. In this way, the less elastic parts can be deformed and serve as a crumple zone to reduce the possibility of breakage of cam fragments by the pump housing causing injury or damage.

As in 10 are shown, the reinforcing ribs of the rotor 402 aligned and the ribs of the rotor 404 are aligned, but the reinforcing ribs of one rotor are not aligned with the reinforcing ribs of the other rotor. The reinforcing ribs of the cams of the one rotor may be aligned because they have an approximately fixed relative relationship and do not collide. However, it should be understood that the alignment of the direction of reinforcement of the nubs of a single rotor is not a requirement to provide crumple zones for absorbing the stored energy of rotors, as described above.

It can be seen that the present invention provides rotors with a high strength to weight ratio. In the drawings, the pump chambers house two rotors having intermeshing cams, but the invention is equally applicable to other configurations, such as rotors having three or more cams.

Claims (21)

A vacuum pump rotor for use in a vacuum pump having a Roots pump mechanism, the rotor having at least two hollow cams, each cam having an outer wall defining a cam profile, a cavity generally within the outer wall, and at least one reinforcing rib located in the cavity is arranged to withstand a stress of the cam, which is generated during the rotation. The rotor of claim 1, wherein the or each reinforcing rib extends around an inner wall of the cams. A rotor according to claim 1 or 2, wherein the or each reinforcing rib has a variable extent and is distributed within the cavity in response to the variable stresses acting on the cam during operation. A rotor according to any one of the preceding claims, wherein the outer wall has a variable thickness and is thicker at a radially inner portion than at the cam tip. A rotor as claimed in any preceding claim wherein the outer wall of the cams is of a thickness such that the cams deform under centrifugal loading when the rotor is rotated during operation and the deformation is greater than manufacturing tolerances. The rotor of claim 5, wherein the cam profiles have an optimal configuration in a first state in which the rotor is rotated in operation and a second state when the rotor is not rotated and the cam profile is not in an optimal configuration, and wherein the cam deforms from the second state to the first state when the tip speed of the cams is greater than 100 m / s. A rotor according to any preceding claim, wherein a ratio of the wall thickness to a radius at the cam tip is less than 1:20. A rotor according to any preceding claim, wherein the thickness of the wall is less than 5 millimeters and the radius of the cam tip is at least 100 millimeters. A rotor according to any preceding claim, wherein each hollow cam comprises a plurality of hollow cam portions which are connected in axial sequence along the rotor and together form the cam. The rotor of claim 9, wherein each of the hollow cam portions has a flange extending circumferentially and radially inwardly around at least one axial end of the portion to connect adjacent portions. A rotor according to claim 10, wherein one or more holes are provided in the flanges for securing the hollow cam portions together by fasteners. A rotor according to any preceding claim, wherein each cam further has two end surfaces for closing the cavity at each axial end of the cam. A rotor according to any preceding claim, wherein the rotor has a shaft and the cams comprise means by which the cam can be secured to the shaft, the cams and the shaft being shaped to have a generally continuous profile of the at least two Cam and the shaft form. The rotor of claim 13, wherein the hollow cam and the shaft of the rotor are adapted to mate by means of a dovetail or similar connection such that the radial movement of the hollow cam portion with respect to the shaft of the rotor is minimized. A rotor according to any preceding claim, wherein vent means are provided to equalize the pressure in the cavity substantially with respect to the pressure outside the hollow cams. The rotor of claim 15, wherein the venting means comprises a filter to filter out deposits of gas entering the cavity through the venting means. Vacuum pump with a rotor according to one of the preceding claims. Vacuum pump according to claim 17, comprising a plurality of pump stages, each having a pump chamber and at least two cams. Vacuum pump according to claim 17 or 18, wherein at least one of the pump stages comprises a cam having a plurality of cam portions which are connected to each other in axial sequence. The vacuum pump according to any one of claims 17 to 19, wherein the reinforcing ribs in the cam cavities respectively extend in radial planes relative to the axes of the rotor shafts, and the radial planes of the lobes of one rotor are not aligned with the radial planes of the other rotor. The pump of claim 20 wherein the portions of the cams between radial planes are configured to deform as the portions of the cams aligned with the radial planes impact thereon to absorb energy of the rotors in the event of a random rotor impact.

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